In recent years, there is a growing trend toward protection of the environment and savings of energy resources, and, in the automobile industry, there has been fierce competition of the development of a hybrid electric vehicle (HEV) using both gasoline and a motor driven by electricity and an electric vehicle (EV) driven only by a motor, instead of the development of a conventional automobile using gasoline as a fuel.
The performance of the electric vehicle heavily depends on the characteristics of the storage battery used as a source for supplying electric energy to the vehicle, and therefore battery manufacturers in cooperation with the respective electric vehicle manufacturers are competing fiercely for the development of a lithium-ion secondary battery.
The lithium-ion secondary battery has a feature such that it is light-weight and exhibits high output, and hence is the most promising storage battery for use in electric vehicle.
However, with respect to the weight energy density of the lithium-ion secondary battery, the theoretical upper limit is considered to be about 400 Wh/kg, and the actually achieved weight energy density is as small as about 100 Wh/kg.
It is said that for widely spreading the electric vehicle, the electric vehicle requires a weight energy density of about 500 Wh/kg, and the development of an innovative battery expected to have a weight energy density larger than that of the lithium-ion secondary battery, which is the storage battery currently mainly studied and developed, is desired.
One of the causes of restricting the weight energy density of the lithium-ion secondary battery is a positive electrode material formed from a lithium-containing transition metal oxide, such as lithium cobalt oxide. The transition metal element as an element constituting the positive electrode material is a heavy metal, and a storage battery having the transition metal incorporated thereinto is increased in weight, so that the storage battery is reduced in weight energy density.
For removing this disadvantage, a metal air battery utilizing oxygen in air as a positive electrode material and a metal as a negative electrode material has attracted attention.
Further, when the metal air battery is used in electric power storage, it is expected that the weight reduction due to the use of the metal air battery can reduce the cost, and therefore the metal air battery is thought to be promising.
Like the lithium-ion battery, the metal air battery is needed to be increased in the number of lamination, area, and capacity for surely achieving the practical output.
With respect to the metal air battery, only a metal air primary battery has been practically used, for example, representatively, a metal air battery using zinc is used as a power source for a hearing aid, and a rechargeable metal air secondary battery has not yet been put into practical use.
For example, PTL 1 discloses a lithium air battery having a construction using as a catalyst a composite oxide having an alkali metal or an alkaline earth metal, a transition metal, and oxygen.